The twelfth son of Johann II, Duke of Saxe-Weimar (1570-1605) and his wife Dorothea Marie of Anhalt (1574-1617), Bernard of Saxe-Weimar studied at the University of Jena in his youth before joining the court of the Elector of Saxony, John George I. Upon the outbreak of the Thirty Years War in 1618, he enlisted to fight against the Habsburg forces and proceeded to fight in numerous battles, serving with German, Danish and Swedish forces. As a commander in the Swedish army, Bernard led a successful campaign to re-invade Bavaria in 1633 and was rewarded with the former Bishoprics of Würzburg and Bamberg and the title of Duke of Franconia. He later entered the service of the French army as well as acting as general-in-chief of the forces of the Heilbronn League of Protestant princes. In 1638, Saxe-Weimar embarked on his most successful campaigns at Rheinfelden, Wittenweiher, and Thann, capturing the cities of Rheinfelden, Freiburg, and Breisach. He died, apparently of a fever which had spread through his camp, in 1639.